Membrane separation techniques using a porous membrane having a selective permeability have made remarkable progress. The membrane separations are used in various fields such as production of drinking water, ultrapure water, drugs, and medicines, and sterilization and/or finishing of brewing materials or brewed products so as to achieve practical development in a series of purification system including separation, cleaning, sterilization, and others. In these fields, due to highly leveled requirements on fine treatment (advanced treatment) of water, safety improvement, and separation accuracy, development in porous membranes to respond such requirements has proceeded.
In water treatment applications such as water purification processing, since porous membranes to be employed in many cases are required to treat a large amount of water, the porous membranes are generally hollow fiber membranes that can increase effective membrane area per unit volume. Further, the hollow fiber membranes having increased water permeability can reduce necessary membrane area, can decrease installation cost due to smaller size of equipment, and can achieve advantages in view of membrane exchange costs and sizes of plant areas. However, there is a problem that water permeability of membranes gradually decreases along with continuous filtration operation due to membrane fouling by various substances contained in water to be treated.
Filtration of raw water rich in pollution substances generally causes sediment (clogging) of organic substances contained in the raw water on surface or internal portion of the membrane during continuous filtration. The sediment build-up increases filtration resistance of the membrane, resulting in deteriorated filtration capacity. Accordingly, the filtration operation is usually suspended in the middle of operation, and the membrane is washed by flushing to peel off the sediments with water at a high flow rate, by air-scrubbing to peel off the sediments by bubbles touched onto membranes, or by reverse-cleaning with flow of gas such as air or treated water into the opposite direction from filtration to clean the membrane. Furthermore, periodic chemical cleaning also can be performed in order to maintain high filtration capacity of membrane. Although flushing and air-scrubbing are effective in cleaning of membranes, conventional membranes cannot achieve satisfactory permeability because of high accumulation of sediments attached to the membrane over time even after being cleaned with these cleaning means.
Separation membranes containing vinylidene fluoride resin have been attracted attention as a material having high chemical resistance and high physical strength, and physical durability so as to have a tendency to expand the future applications. However, vinylidene fluoride resin membranes are easy to be fouled due to the hydrophobic material. Accordingly, it is necessary to avoid fouling (clogging) of the vinylidene fluoride resin membranes by organic substances.
So far, there is proposed a method of immersing a filtration membrane being composed of a vinylidene fluoride resin in an ethylene-polyvinyl alcohol copolymer solution in order to impart hydrophilicity to the filtration membrane (Patent Document 1: JP Patent Application Laid-Open No. 2002-233739). However, since the hydrophilic treatment by means of the immersion cannot achieve sufficient hydrophilization of the membrane to the internal portion, such immersed membrane does not maintain water permeability once in a dry state. As a result, the vinylidene fluoride resin membranes need to be kept immersed in a liquid or to be treated by a wetting operation before use, so as to cause problems in usage of the vinylidene fluoride resin membrane in view of cost as well as handleability.